Oligocene

Hyaenodon horridus, a large carnivorous mammal from the White River Oligocene of South Dakota. Deposits containing Hyaenodon are found in Badlands National Park. (Source: UCMP)

Most of this article on the Oligocene Epoch was written by P. D. P and Brian R. Speer.

Introduction

The Oligocene epoch is part of the Tertiary Period in the Cenozoic Era, and lasted from about 33.7 to 23.8 million years ago (mya). The Oligocene is thus a relatively short span of time, though a number of major changes occurred during this time. These include the appearance of the first elephants with trunks, early horses, and the appearance of many grasses: plants that would ultimately produce vast tracts of grasslands in the following epoch, the Miocene.

Life of the Oligocene

As a result of the cooling trend prevalent throughout the Oligocene period, the lives and habitats of many organisms were directly affected. In the Seas of the world, marine biotic provinces became more fragmented as sea dwellers capable of withstanding cooler temperatures congregated to places further from the warmer equator, where other species could better survive. The cooling trend was also responsible for the reduced diversity in marine plankton, the foundation of the food chain.

The chart shows the major subdivisions of the Paleogene, the first portion of the Tertiary Period, including the Oligocene. The Oligocene Epoch is part of the Cenozoic Era. (Source: UCMP)

On land, mammals such as horses, deer, camel, elephants, cats, dogs and primates began to dominate, except in Australia. The continuation of land mammal faunal migration between Asia and North America was responsible for the dispersion of several lineages onto new continents. Early forms of amphicyonids, canids, camels, tayassuids, protoceratids, and anthracotheres appeared, as did caprimulgiformes, birds that possess gaping mouths for catching insects. Diurnal raptors, such as falcons, eagles, and hawks, along with seven to ten families of rodents also first appeared during the Oligocene. The "bulk feeding" in the open grasslands and savannas that occurred in this period resulted in the increase of general herbivore size. As an example, ungulates continued to get larger throughout the Oligocene period.

The early Oligocene period was marked by a multitude of different events ranging from the appearance of new groups such as elephants to the decline in taxonomic diversity in middle- and high-latitude forests. "Micro-mammals" experienced a period of diversification, as did the marsupials in Australia. This period was also marked by a relative free change of animals among northern continents, as evidenced by the similarity in vertebrate faunas.

In North America, the cricetids (voles and hamsters) first appeared while the mesothermal dicotyledons (a group of flowering plants) went extinct. South America became dominated by forests, and the first primates appeared in Africa. Primates found in Southeast Asia during this period represent primitive members of the New World and Old World higher primates.

In Western Europe, an extraordinary, sudden change in the fauna, known as the Grand Coupure, occurred. This event involved the immigration from areas to the east of many new taxa, artiodactyls and perissodactyls in particular (e.g. rhinocerotoids, chalicotheriids, anthracotheres, and tayassuids), and the extinction of many Eocene genera and species. At least 17 generic extinctions, 20 first appearances, and 25 unaffected genera of mammals are represented across the Eocene-Oligocene boundary in Western Europe.

On a global scale, broad-leaved evergreen vegetation became restricted to 35 degrees latitude around the equator, and megathermal, multistratal vegetation was confined to 15 degrees latitude around the equator. Broad-leaved evergreen plants became increasingly confined to lower latitudes in Eurasia, and microthermal, broad-leaved forest became common over large regions of the Northern Hemisphere.

The mid-Oligocene period was marked by a worldwide marine regression, this included the decline in the total number of marine species. On land, the first of the open grassland faunas appeared in Mongolia while in North America, microthermal broad-leaved deciduous forests extended further into southern regions typified before by evergreen species and for the first time in history covered vast regions of the Northern Hemisphere.

The late Oligocene period was marked by the expansion of grasslands and prairies that were intimately linked to the expansion of grazing animals. Grasses and composites increased in abundance on the global scale, and humid forests became increasingly common in the southern parts of South America. Horses experienced a period of diversification; anatomical modifications in horses indicate an increase in cursoriality compared to more primitive ancestors. Primitive beavers appeared, and the earliest of the New World monkeys lived in South America.

The late Oligocene Deseadan record includes two major groups that are thought to represent early waif dispersals from other continents. One of these, the caviomorph rodents (e.g. Porcupines, capybaras, chinchillas, and a wide assortment of smaller forms), was the only group of rodents in South America until the Plio-Pleistocene. They diversified into 16 families, only two of which are now extinct. The second group of early immigrants was the primates.